1. Field of the Invention
The present invention relates to a stepper motor controller for conveying an original document in a facsimile device, and more particularly to a stepper motor controller capable of accelerating a stepper motor in stages without causing the stepper motor to degenerate.
2. Description of the Prior Art
Facsimile devices read image data from a document. The image data in a main scanning direction Is read using a scanner device equipped with charge coupled devices (CCDs) while conveying the document in an auxiliary scanning direction. Since one line data on the document is read during a fixed reading period, a stepper motor is controlled so as to convey the original document one line for each reading period. Accordingly, the stepper motor is driven at a uniform speed from the moment it is started.
It is desirable to further increase the scanning speed for this method of scanning. To achieve this, it is necessary to increase both the scanning speed of the scanning device and the conveying speed of the stepper motor. Increasing the speed of the scanner is accomplished by improving the performance of the CCDs provided in the scanning device. However, increasing the conveying speed is difficult if the stopper motor must be started at a high speed. Therefore, a driving control is provided to drive the stepper motor at a low speed when the motor is started, and through-up control (accelerating control) is subsequently performed to gradually drive the motor at higher and higher speeds.
When performing through-up control, image data on the document must be read during acceleration. Therefore, the stepper motor is controlled so as to always convey one or a plurality of lines of the document in a duration of time that is an integral multiple of the reading period performed by the scanning device. As described in Japanese Patent Laid-Open Publications (Kokai) Nos. HEX-6-233072, HEI-7-212525, and HEI-7-307837, a document is read even during through-up control, by generating scanned data for a number of lines equivalent to the number of lines conveyed, although the data for every one or plurality of lines conveyed may be scanned a plurality of times.
However, in order to convey the document one or a plurality of lines over a time duration that is an integral multiple of the scanning period, accelerating control of the stepper motor cannot be performed gradually or continuously, but must be performed in stages having large differences in speed.
For example, if the reading period of a scanning device is 2 ms and one line of the document is conveyed by 4 step drives of the stepper motor, acceleration of the stepper motor must be executed in stages having driving frequencies 250 pps (pulses per second). 500 pps, 667 pps, and 1,000 pps, as shown in FIG. 5. When the stepper motor is driven at 250 pps, the duration of time required to convey the document one line is 8 times the reading period. The duration of time required to convey the original document one line at 500 pps is 4 times the reading period; at 667 pps, 3 times the reading period; and at 1,000 pps. 2 times the reading period.
Since the amount that the document is conveyed in one step of the stepper motor is always the same, the driving frequency and the driving speed of the stepper motor are proportional. Hence, as the driving frequency increases, so does the driving speed.
FIG. 6 is a graph showing the speed of a stepper motor that is accelerated by stages having the above-mentioned driving frequencies. The speeds shown on the graph are the angular speeds of the output shaft in the stepper motor that were measured using a non-contact laser Doppler method. In order to facilitate understanding, higher speeds are indicated at higher levels in the graph.
As shown in FIG. 6, the stepper motor is accelerated in stages from 500 pps to 667 pps. However, when accelerated from the driving frequency of 667 pps to 1,000 pps, the stepper motor cannot follow that acceleration and falls out of step or degenerates. Hence, when a stepper motor is accelerated in stages of driving frequencies greatly different in speed, the stepper motor cannot keep up and degenerate.